I can't point to any particular cause.. Do you see tiny "tunnels" in the leaves in the damaged are ? .. if so , it could be a borer of some sort. I think it is most likely just the leaf gets weather or mechanical damage of some sort in those spots and it cuts off the circulation in the veins of the area. It dries out and dies.. I don't think it is anything to worry about unless you see a lot of damage. I think a bit in some leaves is likely normal.. I occasionally see the same damage in chard I grow but it is never enough to be of concern; or from any obvious cause.

Well it looks like the chard I grow, and I get the same blotches on it, as well as beet greens (which are a chard relative). I have never figured it out, but just discard the browned spots.

However, I was having some issues with too little boron giving me beet scab, so I have added micro nutients this year, and we will see if these spots go away too. If so, I would have to guess the spots are related to that too.

Looks like a cabbage worm..They hit our chard , spinnich, radish and the beats this year..First time in 12 years for us..You may need to spray with a product called BTK. Its avaiable here in Canada..Have a look at the dark spots..does it look like the leaf is blistered at all? if so lok for tiny worms..